No no. I cant beleive I am arguing with the King of mechanics here. Its abit like turning my light sabre onto Obi One 
I mean the sharp/short 'shock' of the hammer releases the 'stichon'/where it has bonded when left. Ive done this with a steel frame before. The key is to keep twisting before you knock it down.
PMSL @ Hora 
O?T- PeterPoddy when are you coming upto the Peaks then for a ride?
Despite the replies, Hora's suggestion worked for me once. Stuck an old saddle on, firm tap upwards with a mallet then turn & repeat. Gradually worked it outwards. Post was a bit shagged though.
Despite the replies, Hora's suggestion worked for me once
You are knocking up tho. He is advocating banging it down.
To be fair, Hora is right, shock often works, I would only move it downward the once though!
Not banging. The metal on metal resonates/'shock's the surface is the best way of describing it.
however I once banged as hard as possible against some stuck-fast rotor bolts in my King rear hub and hey presto- THEY LOOSENED! 
Hora, if you're hitting something with a hammer hard enough to loosen it you are banging. Or is it called aggravated tapping these days?
Technically, that's what's known as a 'slight adjustment with a precision instrument'
No- it'll free it. You are not literally hitting it down into the frame like a tentpeg! Hit/Twist/hit/twist...Just enough friction to break the 'bond'!!
But it's not "bonded", it can twist, it's a mechanical interference that's causing the problem. Sure a good bang often shocks a corroded bond into giving up, but once the post moves in the hole you want to be knocking it out of the frame, not down into it 
Could you leave the bike in a warm room and over time let the steel expand and catch up the alloy and pour cold water over the post?
Would a blow torch to gently warm the frame damage the bike material?
I can't believe I'm about to defend Hora, but any 'doubters' should look at impact drivers, they work the same way.
We had on like this once, the alloy had a grove cut into it by a burr inside the post, be careful by the time ours came out there was a bit of 'ovalisation' at the top! If this is the case with yours just see it it will pull up in one position better than others, and means that small movement twist, one way than the other, whilst pulling, might be better than continuously twisting one way whilst pulling. Good luck, let us know the result
I can't believe I'm about to defend Hora, but any 'doubters' should look at impact drivers, they work the same way.
Impact drivers are there to release bonded fasteners, not ones that are loose enough to rotate but are catching on something and not sliding out fully. Same applies to seatposts
So we are assuming that 'something' is caught in the frame?
I thought modern manufacturers would of sorted that kind of problem, maybe not 
But I cant see how if the post has gone down the seat tube, it cant get back out, if you suggesting a bur then surely that would of been pushed further down the seattube when the post was inserted?
If its a case of the post was forced in then I take it all back, hard in will of course equal hard out, but more so.
Bilt Hambers Farrosol release oil
That's pi$$ poor, the frame's haven't been out for long. I bet you didn't get that problem with your Thorn James?
Can we have a diagnosis and repair rather than a witch hunt?
Use window cleaner - ammonia dissolves aluminium oxide.
This really does work - spray a bit twist, spray twist, maybe spray some wd40 to lube it up too (but it'll need continuous applications of ammonia)
This is what you need, its a slide puller or hammer, drill through the seat post and attatch bar, and then use the hammer to shock the post out. This set has internal and external jaws for gripping bearings etc so they may grip the post. The post turns so it isn't seized it sounds like its stuck on a lip or ridge. You can also hire these from tool centres.
http://www.machinemart.co.uk/shop/product/details/cht251-10-pce-slide-hammer-puller-set
These will pull a hub of a car they should pull the post out of the seat tube!
Try the non-destructive way first!
variation on inbred's suggestion. Had a similar situation on a kids Hotrock. The post would turn but not come out. We tapped a thread on the inside of the post, screwed in a slide hammer with a bolt welded onto the end, a couple of blows an out it came. There were some serious grooves on the post when it came out - must have been some debris on the welds that locked the seatpost in.
The ONLY time my method hasnt worked has been a carbon post in a steel frame as the bond is probably tighter but more importantly....the carbon tends to 'absorb' the shock waves rather than transmit the 'shock' to the sticking-surface.
I feel sorry for the pig , no wonder its blue
Now, if you Waxoyl'd it from new..........[opens a can of worms for the nay sayers to dine on]
Well, did ya get it out?
There is only one way to settle this....
FIGHT!
use or make a slide hammer
Not yet but I've been away - will have a good go at it next weekend
It's now been at 18 Bikes for 2 weeks - hopefully they can get it out without damaging the frame
Plusgas.............loads
If all else fails....cut the post across the top....then cut sections down the inside of the post until you can fold a section out........then the rest will follow
Bang it into the frame - it worked on stumpjumper which hadn't been touched for over a year.
We had tried everything -plusgas, kettle trick, hole drilled & connected to a 10ft scaffolding pole to twist etc. All didn't work.
I was actually selling the bike & the guy was literally 10 mins away - final option was the hammer into frame trick - 30 seconds later - new post in frame & as good as new.
Any news James?
Yes! Had to get a new frame under warranty. There was a burr of metal down the seat tube stopping the post from coming out but allowing it to twist.
Great service from 18 Bikes/Ragley/Hotlines who supplied new frame, post and saddle and swapped all the bits over.
I love a happy ending, it makes me want to cry 
All's well that ends well, good to see the warranty works on these new breeds of pig iron
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